To increase the amount of information that can be transmitted per unit of time, recent optical communications use wavelength division multiplexing, in which plural wavelengths (or frequencies) of light, each carrying a different signal, propagate through a single transmission line. In wavelength division multiplexing, plural wavelengths of light are mixed at the inlet of the transmission line, and the mixture of light is separated into the plural wavelengths of light at the outlet. This process requires an optical multiplexer and an optical demultiplexer, or wavelength filters (or frequency filters).
A type of demultiplexer currently used is an arrayed waveguide grating. An arrayed waveguide grating typically uses a quartz glass optical waveguide. With this construction, it is currently necessary to make the device as large as roughly several square centimeters to adequately decrease the loss of light. Taking into account the above-described situation, research has been conducted on the miniaturization of demultiplexers by using a frequency filter composed of a photonic crystal.
A photonic crystal is a functional material having a periodic distribution of refractive index, which provides a band structure with respect to the energy of light. It is particularly featured in that it has an energy region (called the photonic bandgap) that forbids the propagation of light. Introduction of an appropriate defect into the distribution of refractive index in the photonic crystal will create an energy level (called the defect level) due to the defect within the photonic bandgap. This allows only a specific wavelength of light having an energy corresponding to the defect level to exist within the wavelength range corresponding to the energy levels included in the photonic bandgap. Forming a linear defect in the crystal provides a waveguide that propagates light having a specific frequency, and forming a point-like defect in the crystal provides a resonator that resonates with light having a specific frequency.
The Japanese Unexamined Patent Publication No. 2001-272555, which is called “Patent Document 1” hereinafter, discloses a two-dimensional photonic crystal with cylindrical holes periodically arranged in a triangular lattice pattern to provide a periodical distribution of refractive index, in which a linear zone devoid of cylindrical holes is formed as a waveguide ([0025], FIG. 1) and a point defect is formed in proximity to the waveguide ([0029], FIG. 1). Patent Document 1 also includes the analysis of an embodiment of a point defect that is formed by increasing the diameter of the periodically arranged cylindrical holes. This construction provides a demultiplexer for extracting a ray of light having frequency equal to the resonant frequency of the point defect from the light propagating through the waveguide, and a multiplexer for introducing a ray of light having frequency equal to the resonant frequency of the point defect into the waveguide.
The Japanese Unexamined Patent Publication No. 2003-279764 (“Patent Document 2”) discloses a cluster defect created by making defects of two or more pieces of modified refractive index areas adjacent to each other within a plurality of modified refractive index areas forming a periodical distribution of refractive index. The defects of the modified refractive index areas are formed by making the refractive index of the desired part of the modified refractive index areas different from that of the rest of the modified refractive index areas. A defect having a refractive index higher than that of the rest of the modified refractive index areas is called the donor type defect, whereas a defect having a lower refractive index is called the acceptor type defect. The defect disclosed in Patent Document 1, which is created by enlarging the cylindrical hole, is an acceptor type defect, whereas a defect created by not providing the modified refractive index area is a donor type defect. The cluster defect and a point defect created by the absence of a single piece of modified refractive index area are generally called the “point-like defect.”
Patent Document 2 also discloses a two-dimensional photonic crystal having an in-plane heterostructure provided with plural forbidden band regions, each of which has modified refractive index areas distributed with a different cycle and a point-like defect present within each forbidden band region. This construction enables the point-like defects of the same shape to resonate with different frequencies of light according to the different cycles of the modified refractive index areas.
The wavelength multiplexer/demultiplexer disclosed in Patent Document 1 or 2 introduces or extracts light between the waveguide and the outside through the point-like defect. The Japanese Translation of PCT International Application No. 2001-508887 (“Patent Document 3”) discloses a two-dimensional photonic crystal having a point-like defect located between a pair of linear waveguides (see FIGS. 3 and 8 in Patent Document 3). This construction allows a ray of light having frequency equal to the resonant frequency of the point-like defect to be introduced from one waveguide to the other, thus functioning as a multiplexer. It is also possible to use it as a demultiplexer in which a ray of light containing plural frequency components superimposed propagates through one waveguide and a ray of light having a specific frequency is extracted from that light and introduced into the other waveguide.
If the distance between each waveguide and the point-like defect is reduced so that light can be transferred between the two elements, the transfer of light also takes place between the two waveguides. Therefore, in the construction disclosed in Patent Document 3, a ray of light that contains frequency components different from the predetermined frequency is permissible to be transferred at a position different from the point-like defect. This causes the problem of a signal crosstalk.
Though the description thus far has focused on optical multiplexers and demultiplexers, it should be noted that the above-described problem also applies to multiplexers or demultiplexers for electromagnetic waves. Furthermore, optical frequency filters (or wavelength filters) or electromagnetic wave frequency filters that do not use a two-dimensional photonic crystal also suffer from the same problem. In the following description, the term “electromagnetic wave” should be understood as inclusive of light.
The present invention aims to solve the above-described problem, and one of its object is to provide an electromagnetic wave frequency filter capable of efficiently transferring an electromagnetic wave having a predetermined frequency between two waveguides.